Meeting Abstract
In many animal species, individuals of one sex are larger than the opposite sex within a population. This phenomenon, called sexual size dimorphism, poses many mechanistic mysteries because it can arise from a multitude of factors. The sexes can also differ in relative proportions of body parts, called shape dimorphism, regardless of whether body size does or does not differ. Although the sexes often appear virtually identical during early development, they usually undergo highly divergent growth patterns to achieve different adult sizes and shapes. We conducted a study with wild six-lined racerunners to investigate the mechanisms that cause sexual size and shape dimorphism. Adult six-lined racerunners were captured in the wild, and limb, head, and body measurements were taken. We also measured sprint-speed performance to determine if sex differences in morphology manifested in sex differences in performance. Although the sexes did not differ in overall body size, univariate and multivariate analyses showed that males have larger heads and longer limbs relative to their body size than females. This shows that male and female six-lined racerunners exhibit patterns of sexual shape dimorphism, but not sexual size dimorphism. We also found that despite a sex difference in limb morphology, there was no sex difference in sprint speed. This suggests that limb divergence does not result in performance dimorphism, perhaps due to the larger head size of males.
